<input type="file" id="files" name="files[]" accept=".txt"/>

function processNumbers(numbers) {
  console.log(numbers);
  document.body.append(
    JSON.stringify(numbers)
  );
}

files.addEventListener('change', async ({target:{files:[file]}}) => {
  if(!file) return;
  const text = await file.text();
  const numbers = text
    .trim()
    .split(/\s+/)
    .map(Number);

  processNumbers(numbers);
}, false);